1. Field of the Invention
The present invention relates in general to a weft picking device for an air jet type weaving loom in which a weft yarn is adapted to be blown into the warp shed by means of air jet action, and more particularly to an improvement in a weft inserting nozzle of the weft picking device.
2. Description of the Prior Art
Many types of weft inserting nozzles are known and used in the field of air jet type weaving looms. One of them is of a type comprising generally a weft inserting tube through which is passed a weft yarn, with a first chamber section coaxially surrounding the tube to form an annular air jet opening about the tip of the tube, and a second chamber section extending coaxially and downstream from the air jet opening. In this type of nozzle, there are two ways for increasing the traction force applied to the weft yarn for the weft yarn picking. One is to increase the velocity of air ejected or jetted from the air jet opening and the other is to increase the longitudinal length of the second chamber station. The former technique sometimes causes an easy breakage of the weft yarn because of difficulty in setting the air velocity at the optimum value. Thus, it is generally recognized that the latter way is more practical.
However, increasing the longitudinal length of the second chamber section tends to bring about a slack of the weft yarn in the nozzle, originating from a difference in traction force between the force applied to the weft yarn within the nozzle and the force applied to the weft yarn just issued from the nozzle. In fact, while flowing through the elongated second chamber section, the air is gradually accelerated steadily pulling the weft yarn out of the weft inserting tube. Upon ejection from the nozzle, this air collides against the surrounding air thus reducing its velocity, so that the weft yarn issuing from the nozzle decreases its velocity. Thus, the weft yarn in the second chamber section is subjected to contraction, producing the undesired slack. Further, increasing the longitudinal length of the coaxial downstream chamber tends to cause the weft yarn in the nozzle to untwist, especially when it is exposed to a high velocity air throughout a long length thereof. Furthermore, this technique induces a possibility of back flow of air toward the first chamber section, which of course prevents the weft yarn from being optimally picked into the warp shed.